Equalisation arrangement

Abstract: An equalisation arrangement compensates for multipath phase and amplitude distortion effects in a transmission channel by first measuring such distortion effects as they occur over time in a calibration phase, thereby obtaining phase and amplitude compensation factors, then applying these factors at the appropriate times during a subsequent data transmission phase to a correcting means situated in the signal path. Calibration is achieved by feeding a received calibration signal to an amplifier, preferably a successive detection logarithmic amplifier (26), comparing the phase of the limited linear output of the logarithmic amplifier with a delayed version of itself (30, 28), and using the amplitude information (39) in the logarithmic output (72) of the logarithmic amplifier (26) and the phase-change information (35) resulting from the phase comparison to compute phase and amplitude compensation coefficients. In the data transmission phase, the compensation coefficients are applied to amplifiers and multipliers, etc (56, 55), in the correcting means.

Description: The invention relates to a method of correcting for amplitude and phase distortions in a transmission system due to, for example, multipath propagation. Communications systems employing radio, ultra-sonic, infra-red or optical transmission are well known in the art. It is a common problem in such systems, however, that under certain circumstances propagation of the transmitted signal takes place not only along one path, but along two or more paths simultaneously. When this occurs, multiple signals are received at the receiving antenna at times which depend on the length of the individual propagation paths. The result may be distortion in both the phase and amplitude of the received signal. Such a situation is illustrated in Figure 1. Line (a) of Figure 1 shows the situation at a time t 1 as a receive antenna picks up a transmitted signal 10 of wavelength π and zero-crossing phase of 0, π, 2π, etc., relative to the ordinate 11. This is a signal from a direct propagation path. Subsequently, at a time t 2 , the same signal but from a different propagation path arrives at the antenna and interferes with the already received signal in (a). This newly arrived signal is shown as signal 12 in line (b) of Figure 1. The result of the superimposition of the two signals 10 and 12 is the dotted waveform 13 and this waveform can be seen both to have a larger amplitude than signal 10 and to bear a different zero-crossing phase relationship with it, namely  1 , π +  1 , 2π +  1 , etc. Finally, due to a further propagation path, a third signal 14 arrives at the antenna (see line (c)) and this interferes with the composite signal 13 to produce a final composite 15 having still greater amplitude, due to constructive interference, and a phase relationship of 2, π + 2, 2π + 2, etc, relative to signal 10. Thus it can be seen that the effect of multipath propagation is to distort the amplitude and phase characteristics of the received signal with respect to the transmitted signal. Note that, for simplicity, signals 12 and 14 have been shown to have the same amplitude as signal 10; in reality, their amplitude may well be much smaller due to attenuation factors in transmission, resulting in a composite waveform 15 manifesting amplitude and phase distortions less severe than those shown in line Figure 1. Such distortion can be especially troublesome when digitally modulated signals are being received, as, when the distortion is severe enough, the information relating to one symbol in a data chain can interfere with the information relating to the next symbol in the chain, or even symbols further on. EP-A-0176029 discloses a digital transmission system in which a transmitted signal includes plural pilot signals inserted in line with pilot gaps in the signal. The receiver includes an adaptive IF equaliser arranged for equalising a received IF signal in respect of frequency-dependent group delay/phase and amplitude distortions. The equaliser has amplitude and delay/phase control means. It is an aim of the invention to provide a method for correcting for amplitude and phase distortion effects. According to the invention, there is provided a method of correcting for amplitude and phase distortion effects in a transmission system due to, for example, multipath propagation, characterised by comprising a calibration phase and a compensation phase, the calibration phase comprising the steps of: transmitting a calibration signal comprising a channel sounding pulse transmitted separately in time from data or a preamble transmitted at the head of a data string, the preamble having known phase and amplitude characteristics; receiving the calibration signal and feeding it to an amplifying means, the amplifying means providing on separate outputs signals representative of the amplitude and phase, respectively, of the received calibration signal; detecting in a phase-detecting means a disturbance in the phase of the signal at the phase output of the amplifying means; deriving from the amplitude output of the amplifying means and from an output of the phase-detecting means amplitude and phase compensation signals for use during the compensation phase, the compensation signals being related to disturbances in amplitude and phase as they occur over time, the compensation phase comprising the application of said compensation signals to a correcting means situated in the path of the received signal, such as to compensate for said amplitude and phase distortion effects. The compensation signals may be used to adjust amplitude and phase characteristics of correcting elements within the correcting means. The invention will now be described, by way of example only, with reference to the drawings, of which: Figure 1 shows the amplitude and phase changes in a received signal when the signal is received over a number of propagation paths; Figure 2 is a schematic diagram of a receiver employing a method according to the invention; Figure 3 is a schematic diagram of the correcting block shown in Figure 2. Figure 4 is a schematic diagram of a successive detection logarithmic amplifier as employed in the equalisation arrangement of Figures 2 and 3, and Figure 5 is a graph of output voltage versus input power for the logarithmic amplifier illustrated in Figure 4. A radio receiver employing the method of the invention is shown in Figure 2. In Figure 2, a mixer 22 receives on one input a transmitted RF signal picked up by an antenna 21 and on another input a local oscillator signal supplied by a local oscillator 24. The output of the mixer 22, which is at Intermediate frequency, feeds on the one hand, on a line 23, an equalisation arrangement 40 according to the invention, and on the other hand, on a line 25, a correcting means 50, which lies in the path of the IF signal on its way to further processing such as further amplification, demodulation, etc. The equalisation arrangement 40 consists of a logarithmic amplifier (log amp) 26, to be described in greater detail below, which receives the IF signal from the mixer and provides, on two outputs, control signals 37 to the correcting means 50, both directly on a line 27 and indirectly on a line 35 via a phase detecting means 32. The phase detecting means 32 comprises a phase detector 30 which has two inputs 31 and 33, the first of which, 31, is connected directly to a linear output 29 of the log amp 26, and the second of which, 33, is connected to the same linear output 29 via a delay line 28. The other output 39 of the log amp 26 provides a signal proportional to the logarithm of the input voltage at the input 34 of the log amp 26. The log amp 26 is now described in greater detail with the aid of Figures 4 and 5. Log amp 26 is constituted by what is known in the art as a "successive detection" log amp and the construction of this device is shown in Figure 4. The successive detection log amp 26 consists of a number n of identical linear limiting amplifying stages A 1 to A n connected in cascade. The output of each stage is detected in a detector D 1 - D n , the input 62 to the whole device being also detected in a detector D 0 . The outputs of detectors D 0 - D n are summed in a delay line 64, the delay line serving to compensate for propagation delays through the amplifier/detector chain, and the output of the delay line 64 is taken to a video amplifier 70 which provides, on an output 72, a signal proportional to the logarithm of the input voltage. Finally, a further output 66 is taken from the output of the final amplifying stage A n . This output is the input voltage multiplied by a gain of A n , but limited to the common output limiting voltage of all the gain stages. The operation of the log amp is now described with the aid of Figures 4 and 5. Each stage A 1 , A 2 , etc. is a linear amplifying stage having a gain characteristic as shown at 82 in Figure 5. The characteristic for the first stage A 1 is shown as 82-1, that for the second stage A 2 is shown as 82-2, and so on, up to the characteristic 82- n for the n th stage. Since the individual amplifiers themselves are linear, plotting their gain characteristic as output voltage versus input power in dB will produce not a linear gain curve but an anti-log gain curve, as shown in Figure 5. In addition, each amplifier is arranged to limit at a level 83. Thus now, when a signal appears at the input 62 of the log amp 26, detector D n will be the first detector to become active, owing to the signal at the output 66 being amplified by a factor A n relative to the input 62. The output of detector D n , amplified in the video amplifier 70, will therefore follow the curve 82- n in Figure 5. As the signal at the input 62 rises further, amplifier A n reaches the limits of its linear operation and goes into saturation. This is shown by the line 83 in Figure 5. When this occurs, amplifier A n -1 starts to drive its detector D n -1 into operation and this produces at the output of detector D n -1 an output following the curve 82- n -1. However, this output is now summed with that for detector A n , which is at the limiting level 83, and therefore the total output from the video amp 70 is as shown at 84 in Figure 5. This process continues for all the stages A n to A 1 in turn as the input signal rises, so that the complete characteristic for the whole log amp is the piecewise curve 85 in Figure 5. It will be noticed that this curve is an approximate straight line, resulting in, as is to be expected, a logarithmic gain characteristic for the log amp as a unit. The number of stages used is dictated by the dynamic range required for the log amp. The signal on the output 66 of the log amp (see Figure 4) is not a composite signal, as is the logged detected signal at the output 72, but is an amplitude-limited signal whose main use is the preservation of phase information which is lost in the detecting/logging process. Thus the two output signals on lines 66 and 72 carry, respectively, phase and amplitude information regarding the signal at the input of the log amp. It is evident that, by using linear amplifying stages in the log amp, unavoidable deviations from a true logarithmic curve (straight line) are obtained. However, by restricting the operation of each amplifying stage to only that part of its gain curve in which deviation from linearity does not exceed a certain limit, an acceptable straight line approximation 85 can be obtained. Also, it is not necessary to use a delay line 64 in the summing process if wideband differential amplifiers are used for the amplifier and detector stages. Referring again to Figures 1 and 2, the operation of the equalisation process is as follows: The equalisation arrangement is first calibrated by arranging for a so-called channel sounding pulse to be transmitted to the receiver 20. This transmission enables the characteristics of the transmission channel which is to be used to be ascertained so that phase and amplitude correction can then take place within the receiver. It is assumed that the channel sounding pulse takes the form of a burst of a sinusoidal waveform such as the signal 10 of Figure 1. The channel sounding pulse is received by the receiver 20, converted to IF in the mixer 22 and passed via line 23 to the equalisation arrangement 40, where it is converted into, on the one hand, a log-amplified signal on output 39 and, on the other hand, a limited linear-amplified signal on output 29 of the log amp 26. The limited linear-amplified signal on output 29 is taken to the phase detecting means 32, where it is compared with a delayed version of itself. This is achieved by means of the delay line 28, the phase detector 30 comparing the signal appearing at the input of the delay line with that appearing at its output. The phase detector 30 is so designed, in conjunction with the delay inherent in the delay line 28, that when the received signal is at a nominal carrier frequency, the phase detector 30 is in the middle of its range. Both the output of the phase detector 30 and the log output 39 of the log amp 26 are taken to the correcting means 50, which processes the information contained in these outputs for use during a subsequent data transmission phase. It is now assumed that the transmitted channel sounding pulse arrives a second time at the antenna 21, i.e. at time t 2 (see Figure 1), having travelled along a second propagation path. At the moment of reception of this second input, the IF signal at the output of the mixer 22 will exhibit both a change in amplitude and a change in phase, as illustrated by the waveform 13 of Figure 1. The change in amplitude is passed directly on to the correcting means 50, after being log-amplified in the log amp 26, while the change in phase ( 1 ) is first detected in the phase detecting means 32, as is now to be explained. At the moment of arrival of the limited, linear-amplified IF signal 13 at the input of the phase detecting means 32, this new signal 13 is passed straight to the input 31 of the phase detector 30; however, the input 33 of the phase detector 30 still sees a delayed version of the old, directly propagated, signal 10. The result is that the original phase difference seen by the phase detector 30 is changed by an angle  1 , and this change or discontinuity, together with the change in amplitude obtained from the output 39 of the log amp 26, is recorded in the correcting means 50 along with a record of the time at which these changes took place relative to the initial receipt of the first signal 10 (the directly received channel sounding pulse). At some later time t 3 the channel sounding pulse arrives at the antenna 21 via a third propagation path, giving rise to a second discontinuity in the amplitude and phase (waveform 15 of Figure 1) of the IF signal at the output of the mixer 22. As in the previous case, when this new signal 15 reaches the phase detecting means 32 the input 33 of the phase detector 30 still sees the previous (second propagation path) signal, corresponding to signal 13 in Figure 1, whereas the input 31 of the phase detector 30 sees the new (third propagation path) signal, corresponding to signal 15. This gives rise to a second change  2 , relative to signal 10, which is registered by the phase detector 30. This change in phase, together with the associated change in amplitude, is, as before, correlated in the correcting means 50 with the time of occurrence of these changes. Referring now to Figure 3, the control signals 37 entering the correcting means 50 are first converted to digital form by means of A/D converters 52, 53 and are then processed in a processor 54, where the required coefficients of phase and amplitude for the correction of phase and amplitude distortion effects are computed and used to adjust the gain and phase factors of one or more multipliers and amplifiers 55, 56 in the signal path. The processor also keeps track of the time that elapses between changes in amplitude and phase as seen by the output 39 of the log amp 26 and the phase detecting means 32, respectively. Calibration having been completed, data transmission may now start. During reception of data, the phase and amplitude characteristics of the amplifier(s) and multiplier(s) 55, 56 in the correcting means 50 are adjusted by amounts previously calculated by the processor 54 on the basis of information gleaned from the log amp logging output 39 and the phase detecting means 32, at times previously registered by the processor 54. In this way the effects of multipath distortion are compensated for in the receiver 20. In a second embodiment of the invention (not illustrated), instead of using a channel sounding pulse to effect calibration, a data transmission preamble having known phase and amplitude characteristics is employed at the very start of transmission. Calibration thus takes place before each transmission of data and is used to effect distortion cancellation during the subsequent transmission of the data portion of the transmitted string.

Claims: A method of correcting for amplitude and phase distortion effects in a transmission system due to, for example, multipath propagation, characterised by comprising a calibration phase and a compensation phase, the calibration phase comprising the steps of: transmitting a calibration signal comprising a channel sounding pulse transmitted separately in time from data or a preamble transmitted at the head of a data string, the preamble having known phase and amplitude characteristics; receiving the calibration signal and feeding it to an amplifying means (26), the amplifying means (26) providing on separate outputs (39, 29) signals representative of the amplitude and phase, respectively, of the received calibration signal; detecting in a phase-detecting means (32) a disturbance in the phase of the signal at the phase output (29) of the amplifying means (26); deriving from the amplitude output (39) of the amplifying means (26) and from an output (35) of the phase-detecting means (32) amplitude and phase compensation signals for use during the compensation phase, the compensation signals being related to disturbances in amplitude and phase as they occur over time, the compensation phase comprising the application of said compensation signals to a correcting means (50) situated in the path of the received signal such as to compensate for said phase and amplitude distortion effects. A method as claimed in claim 1, characterised in that the compensation signals are used to adjust amplitude and phase characteristics of correcting elements within the correcting means. A method as claimed in claim 2, characterised by the step of storing quantities corresponding to the magnitudes of the compensation signals during the calibration phase and retrieving those quantities during the compensation phase. A method as claimed in claim 3, characterised by the step of converting (52, 53) the magnitude quantities of the compensation signals into digital form, deriving compensation coefficients from those digital quantities, and storing those coefficients for retrieval during the compensation phase. A method as claimed in any preceding claim, characterised in that the phase-detecting means (32) detects a disturbance in the phase of the received signal by comparing, in a phase detector (30), the phase output (29) of the amplifying means (26) with a delayed version of itself.

Inventors: CHADWICK PETER EDWARD, CHADWICK, PETER EDWARD
Applicants: ZARLINK SEMICONDUCTOR LTD, ZARLINK SEMICONDUCTOR LIMITED
Application year: 1994
Main classification: H04B7
IPC codes: H04B1, H04L27, H04B7